1. Field of the Invention
The present invention relates to highly pure spherical silica glass powder particles, a process for their production and a method of their use.
2. Discussion of Background
Heretofore, it has been common to air-tightly seal semiconductor devices with an epoxy resin composition containing not higher than 70% by weight of silica glass powder.
In recent years, silica glass powder having a high purity has been required as semiconductor devices tend to be highly integrated, since .alpha.-ray particles generated from impurities cause noises. For example, it is required that impurities such as uranium and thorium be not higher than 1 ppb, and impurities such as aluminum, sodium, iron and chlorine be not higher than 1 ppm.
On the other hand, a sealing composition containing a higher content of silica glass powder is desired to improve the strength of the sealing portion in order to cope with the trend for large size semiconductor devices. However, if the content of silica glass powder is simply increased, there will be problems such that the strength of the sealing portion is likely to deteriorate, the air-tightness is likely to be impaired, and a residual stress is likely to be created. To solve such problems, spherical silica glass powder particles are desired.
Heretofore, silica glass powder having a high purity has been produced by pulverization of a natural rock crystal of high guality. However, because of restrictions with respect to the purity of the starting material, the inclusion of impurities during the pulverization step and the availability of the resource, various new processes have been attempted.
One of them is a process wherein silica glass powder having a low content of impurities is produced by using water glass as the starting material. Namely, Japanese Unexamined Patent Publication No. 54632/1984 discloses that highly pure silica glass powder can be obtained by conducting the reaction of water glass with a mineral acid at a hydrogen ion concentration within a certain specific range. However, this process has drawbacks such that a number of washing steps are required in order to remove the impurities, that after the drying, powder particles are likely to aggregate and it is necessary to take an additional step for disintegrating the aggregates to obtain silica glass particles having a proper granularity, and that a temperature as high as at least 1,100.degree. C. is required to obtain silica glass powder.
Further, for the production of highly pure silica glass, a process has been known in which SiCl.sub.4 is used as a starting material. Fine particles of silica glass used for the production of silica glass base material for optical fibers, are produced by using SiCl.sub.4 as the starting material. Their particle sizes are within a range of from 100 to 2,000 .ANG., and it is common to heat-treat them to a size of from 10 to 100 .mu.m for practical use. However, in this process, Cl.sup.- is likely to remain in the glass, which is not desirable for a sealing filler for semiconductor integrated circuits.
In recent years, an attention has been drawn to a sol-gel method wherein an alkoxide is used as a starting material for the production of silica glass having a high purity. Japanese Unexamined Patent Publications No. 205334/1982 and No. 2233/1983 propose a process which comprises spraying a sol solution obtained by the hydrolysis of a tetraalkoxysilane, to obtain silica powder, which is then heat-treated to obtain silica glass powder. According to this process, the size of silica powder particles formed by the spraying, varies depending upon the condition of the sol solution, whereby it is difficult to obtain silica glass powder having a desired particle size.
Heretofore, a process has been known wherein a silicon alkoxide is hydrolyzed in the presence of a basic catalyst to precipitate silica gel particles. However, this process has drawbacks such that the formed particles are primary particles having a size of submicrons or smaller, and their secondary particles obtained by aggregation have poor dispersebility and tend to be not spherical.
Further, Japanese Unexamined Patent Publication No. 176136/1983 discloses a process which comprises adding to an silicon alkoxide from 1 to 3 times by weight of water to prepare a solution, dispersing this solution in an organic solvent which is incompatible with the solution, thereby to precipitate silica gel, and separating and baking the silica gel to obtain silica glass powder.
However, this process has drawbacks such that the yield of spherical silica glass powder particles is not adequately high, and it is difficult to control the size of the powder particles, since it takes a long time for the reaction. Further, the silica glass powder obtained by this process has a smooth surface and thus has a drawback that when mixed with a resin and used for sealing a semiconductor device, the contacting surface with the resin is small, whereby it is difficult to obtain an adequately high sealing strength.